Microbial Growth in the Environment

Botulinum Toxins and Clostridium botulinum

• Clostridium botulinum: G+ rod, spore‐forming, soil‐dwelling, obligate anaerobe.
• Exotoxin causes flaccid paralysis; forms include infant botulism, foodborne botulism, wound botulism.
• Epidemiology: low incidence but fatal if not treated.
• Possible sources of infection: formula, honey, dirt.
• Toxin properties:
  • Botulinum toxins block motor neurons, leading to flaccid paralysis.
  • Spores are heat‑resistant and can survive in foods that are improperly processed (e.g., honey, sausages, and canned foods).
  • The toxin is heat‑labile and can be destroyed if heated at $80^\circ\mathrm{C} \text{ for } 10\ \mathrm{min}$ or longer.

The Terminology of Microbial Control

• Sterilization: destroys all forms of microbial life.
• Commercial sterilization: limited treatment; destroys pathogens but not all bacteria.
• Disinfection: removing pathogens; disinfectant = chemical treatment used to disinfect inanimate objects.
• Degerming: physical removal of microbes (e.g., alcohol swab, soap, hand washing).
• Sanitize: systematic cleansing of inanimate objects to reduce the microbial count to a safe level.
• Antisepsis: removing pathogens from living tissue.
• Aseptic: absence of sepsis; Sepsis = bacterial contamination (decay, putrefaction).
• Bacteriocidal: kills microbes.
• Bacteriostatic: inhibits, not kills, microbes.

Actions of Antimicrobials

• Target: cell wall

  • Function: blocks synthesis; surface breakdown; cell becomes fragile and lysed easily.

• Alter membrane permeability

  • Function: damage to lipids or proteins causes leaks and interferes with growth.

• Damage to proteins

  • Heat/pH can denature enzymes essential for growth.

• Damage to nucleic acids

  • Prevents information transfer for protein synthesis; chemicals, radiation, and heat can produce fatal mutants; halts protein synthesis through action on RNA.

• Effectiveness of Treatment depends on:

  • Number of microbes.
  • Environment (organic matter, temperature, biofilms).
  • Time of exposure.
  • Microbial characteristics (spore‑forming, thick lipid coats, protozoan cysts).

• Ideal antimicrobial agents should be:

  • Inexpensive, fast‑acting, stable during storage.
  • Capable of controlling microbial growth while being harmless to humans, animals, and objects.

• Germicide Effectiveness (levels):

  • High‑Level: kills all pathogens, including bacterial endospores; used to sterilize catheters, implants, heart/lung machines.
  • Intermediate‑Level: kills fungal spores, protozoan cysts, viruses, and pathogenic bacteria (not endospores); used to disinfect instruments that contact mucous membranes but not invasive.
  • Low‑Level: eliminates vegetative bacteria, fungi, protozoa, and some viruses; used to disinfect items that contact skin.

Effectiveness of Treatment Methods

• Factors affecting efficacy:
  • Site to be treated and environmental conditions.
  • Relative susceptibility of microorganisms.
  • Evaluation methods by comparing to phenol (phenol coefficient concept) and an agent’s ability to control microbes.
• Common evaluation methods:
  • Use‑Dilution testing.
  • Disk‑Diffusion testing.
  • Kelsey‑Sykes Capacity Test.
• Use‑Dilution details:
  • Bacterial suspensions added to the chemical being tested.
  • Samples removed at predetermined times and incubated to determine surviving bacteria.
  • Establishes minimum time required for disinfectant to be effective.
• In‑Use Test:
  • Swabs from objects before and after application of disinfectant/antiseptic.
  • Swabs inoculated into growth medium and incubated; growth monitored.
  • Provides accurate determination of proper strength and application procedures for each situation.
• Disk‑Diffusion Method:
  • Filter paper soaked with chemical agent placed on agar plate containing known organism.
  • Death zone around filter paper measured; larger zone = more effective.

Physical Methods of Control

• Methods include:
  • Heat (moist and dry)
  • Cold
  • Desiccation
  • Filtration
  • Osmotic pressure
  • Radiation

Heat Related Methods

• High temperatures:
  • Denature proteins; interfere with cytoplasmic membrane and cell wall; disrupt nucleic acids.
• Thermal Death Point: $T_{ ext{tdp}}$ = lowest temperature that kills all cells in broth in $t=10\ ext{min}$.
• Thermal Death Time: time required to sterilize a volume of liquid at a set temperature.
• Heat sterilization: high heat and high humidity together are most effective; dry heat can be used in other cases.

Moist Heat

• Used to disinfect, sanitize, and sterilize.

• Moist heat denatures proteins.

• Tyndallization (intermittent heating) used for endospore‑forming bacteria.

• Methods include:

  • Boiling
  • Autoclaving
  • Pasteurization
  • Ultrahigh‑temperature sterilization

• Boiling:

  • Kills vegetative cells of bacteria and fungi, protozoan trophozoites, and most viruses.
  • Boiling time is critical; different elevations require different times.
  • Endospores, protozoan cysts, and some viruses can survive boiling.

• Autoclave:

  • Steam under pressure; steam at $100^\circ\mathrm{C}$ under pressure ($P = 15\ \text{psi}$) can reach $121^\circ\mathrm{C}$.
  • Pressure prevents steam from escaping; typically used for 15 minutes to reach surface and kill endospores.
  • Sterility indicators used to verify.

• Pasteurization:

  • Reduces spoilage organisms and pathogens; not sterilization.
  • Classic pasteurization: $63^\circ\mathrm{C}$ for $30\ \mathrm{min}$.
  • Flash pasteurization: $72^\circ\mathrm{C}$ for $15\ \mathrm{sec}$.
  • Ultra‑high‑temperature (UHT): $140^\circ\mathrm{C}$ for <$1\ \mathrm{sec}$.
  • Thermoduric organisms may survive.

Dry Heat Sterilization

• For materials that cannot be sterilized with moist heat.
• Requires higher temperatures for longer times than moist heat.
• Kills by oxidation and by denaturation of proteins.
• Methods: flaming (tubes, loops, needles); incineration; hot‑air sterilization.

Cold Methods

• Inhibits microbial growth, metabolism, and reproduction (bacteriostatic).
• Refrigeration: $4^\circ\mathrm{C}$; most pathogens cannot grow at this temperature.
• Commercial freezer: $-20^\circ\mathrm{C}$.
• Deep‑freezing: $-10^\circ\mathrm{C}$.
• Lyophilization (freeze‑drying): used for bacterial storage.

Other Methods

• Desiccation: absence of water prevents metabolism.
• Filtration: physical removal of bacteria from liquid or air.
  • HEPA filters remove microbes > $0.3\ \mu\mathrm{m}$.
  • Membrane filtration removes microbes > $0.22\ \mu\mathrm{m}$.
• Osmotic Pressure:
  • High concentrations of salt or sugar cause plasmolysis; cells in hypertonic solutions lose water; fungi more resistant than bacteria.
• Radiation:
  • Ionizing radiation (X rays, gamma rays, electron beams): ionizes water to release OH radicals; damages DNA; penetrates deep; useful for sterilizing plastics and some foods.
  • Nonionizing radiation (UV, $260\ \mathrm{nm}$): damages DNA via thymine dimers; repair methods exist; used to sterilize air and surfaces.
  • Microwaves kill by heat; not specifically antimicrobial.

Ultraviolet Radiation

• Lethal wavelength: $260\ \mathrm{nm}$.
• Absorbed by DNA, causes thymine dimers; repair mechanisms exist.
• Used to sterilize air and surfaces.

Review Table 9.4: Principles of Effective Disinfectant

• Questions to consider for any disinfectant:
  • What is it effective against?
  • What is the concentration required?
  • How does organic matter affect efficacy?
  • What is the optimal pH?
  • How long must contact be maintained?

Chemical Agents: Major Classes and Mechanisms

• Phenol & Phenolics:

  • First chemical used to control microorganisms.
  • Action: denature proteins and disrupt plasma membranes.
  • Advantages: stable and persistent; good surface disinfectants.
  • Disadvantages: disagreeable odor; potential side effects.

• Bisphenols:

  • Phenol derivatives with two phenol rings.
  • Examples: pHisoHex© (surgical scrubbing), Hexachlorophene, triclosan.
  • Mechanism: disrupt plasma membranes; broad spectrum; effective against G+ bacteria and fungi.

• Alcohols:

  • Intermediate‑level disinfectants.
  • Denature proteins and disrupt cytoplasmic membranes.
  • More effective than soap for removing bacteria from hands.
  • Examples: $ext{ethanol}, \text{isopropanol}$.

Halogens

• Elements: Fluorine, Chlorine, Bromine, Iodine.
• Iodine:
  • Tinctures (in aqueous alcohol).
  • Iodophors (inorganic carriers): alter protein synthesis and membranes.
• Chlorine:
  • Bleach: hypochlorous acid $HOCl$.
  • Chloramine: chlorine + ammonia.
  • Oxidizing agents that damage cellular materials.

Oxidizing Agents

• Peroxides, ozone, and peracetic acid.
• Kill by oxidation of microbial enzymes.
• High‑level disinfectants and antiseptics.
• Uses: contaminated surfaces; examples: $ext{O}<em>3, \mathrm{H}</em>2\mathrm{O}_2, \text{peracetic acid}$.
• Ozone is also used to treat water.

Surfactants

• Acid‑anionic surfactants: important surface agents in dairy industry.
  • Action: negative (anionic) portion disrupts plasma membranes; broad spectrum.
• Soaps and detergents:
  • Soaps: hydrophilic and hydrophobic ends; good degerming agents but not strong antimicrobials.
• Quaternary ammonium compounds (Quats): low‑level disinfectants; disrupt membranes; versatile for many medical/industrial applications.

Heavy Metals

• Denature proteins at low concentrations; bacteriostatic and fungistatic.
• Examples: silver nitrate (neonatal ophthalmic gonorrhea prevention), thimerosal (vaccine preservative), copper (algal control), silver sulfadiazine (burn treatment).

Aldehydes

• Highly reactive; inactivate proteins by cross‑linking with functional groups (–NH2, –OH, –COOH, –SH).
• Cross‑linking denatures proteins and inactivates nucleic acids; sporicidal.
• Uses: medical equipment.
• Examples: Glutaraldehyde (disinfects and sterilizes); Formalin (embalming and disinfection of rooms/instruments).

Gaseous Agents

• Ethylene oxide: sterilizes heat‑sensitive materials; highly effective (bactericidal and sporicidal).
• Disadvantages: hazardous to people; often explosive; potentially carcinogenic.

Biguanides

• Chlorhexidine: disrupts plasma membranes.
• Use: in some hospital surgical scrubs.
• Advantages: binds to skin and mucous membranes; relatively low toxicity to humans.
• Effective against vegetative bacterial cells (not spores) and fungi.

Enzymes

• Antimicrobial enzymes act against microorganisms.

• Human tears contain lysozyme; digests peptidoglycan cell wall of bacteria.

• Enzymes used to control microbes in the environment:

  • Lysozyme used to reduce bacteria in cheese.
  • Prionzyme claimed to remove prions on medical instruments.

• Review Table 9.5: Chemical Methods of Microbial Control